An air-source heat pump would heat our house and would hugely reduce energy consumption and CO2e emissions, but it would put up our annual energy costs (DHW and space-heating) by over £220; that’s an increase of +30% compared to our current gas-boiler set-up (in 3-bed semi; the EPC is a high-C.)
This is surely a big problem re uptake of HPs – as I’m sure many have already noted.
Quick calculation, in round figures: we use about 7,000kWhrs annually for heating and 2,000kWhrs for hot water. If a heat pump were to reduce this total to about 3,000kWhrs (ie, COP ~3.0), then if the HW were to be provided during 3hrs at nighttime tariff and the necessary electric power/hot water for space-heating were to be evenly-spread over the other 21hrs, using a tariff like Octopus Plus, I get total HW+space-heating costs of about £742; currently our HW+space-heating by gas costs us £517. (I’ve done a spreadsheet.)
Obvs, I’d have to spend a few thousand on the ASHP installation (notwithstanding B.U.S. grant of £7,500); to follow that with a spend of an extra couple of 100 £ every year for ever and a day is a bit off-putting. Probably.
Interested to hear if others have faced this situation and/or done anything about it. Are my assumptions right, eg, heating-output to be more-or-less constant throughout the day and night?
Many installers fit heat pumps with constant output as you call it. The term used is “a fixed set point” often at the upper limit of their range. This is a throwback to old inefficient boilers. Most condensing boilers are set up with a fixed set point near the top of their range. It is inefficient for condensing boilers and worse for heat pumps.
You need weather compensation, and as far as I know all modern heat pumps come with this ability. As the outside temperature fluctuates the output also fluctuates, keeping the flow temperature to the minimum required to heat your home for a specific outside temperature. By keeping down flow temperature you also increase efficiency. This ties in with the need for adequate sized radiators for low temperature heating.
In this country the price difference between gas and electricity is about 2.8 times, allowing for the losses in gas boiler inefficiency. You therefore need a COP of 2.8 to break even on running costs. However if you are able to disconnect the gas and stop the standing charge your savings are greater.
There are suppliers that offer favourable rates to heat pump owners, so again the minimum cop reduces, or preferably the savings increase.
I believe that the biggest impediments to heat pump take up are currently bad publicity in parts of the press, capital cost (even with grant) and availability of skilled fitters, which has an effect on the capital cost.
If heat pumps were installed and configured correctly the running costs would take care of themselves.
Hmmm… thanks. I’ve been running the gas boiler for two winters now with flow temp fairly stable at low-30s deg C; the thermostat in the living room downstairs controls the firing in the usual way. (It goes off a lot, always quite encouraging to see ) Isn’t a good estimate of what we’d need approximately given by
(current DHW+CH) / (COP)
, ie approx 9000/3= 3000kWhrs ? (In round figures; the spreadsheet is more accurate.) Thus, the room-thermostat+current insulation acts in much the same way as HP weather-compensation would do.
If that’s a reasonable assumption, I’d need COP= 4.5-ish to break even… not currently on offer I think.
That depends on whether your condensing boiler modulates and you have thermostats to control that.
Your current settings are probably close to the efficiency claimed by the boiler manufacturer, so reduce your projected electricity consumption by the gas wastage factor, by which I mean if the boiler has a declared efficiency of 90% (exceptional) then reduce your anticipated consumption by 10%.
There are companies that will guarantee a high COP, some give an indicative one and most don’t care. Once the installation is done you only have limited scope to enhance COP, as the installation itself will have a major effect.*
One difference between a low temperature gas system and a heat pump is the required flow rate, which is higher with a heat pump. I can’t say I’ve noticed a difference but if you have very long pipe runs you may get better heating at the extremities of your system with a heat pump and can therefore reduce temperature slightly, increasing COP, or conversely if you have restricted flow anywhere the heat pump will need to work harder and your COP will suffer.
I’ve seen it said that there are no bad heat pumps, just bad installations.
My ASHP was installed June, commissioned in July and has achieved an average COP of 4.2 over 6 months. Not the 4.5 you’re looking for, but better than the average install.
It was well designed and well installed by Heat Geek, but even with BUS grant, it was more than a ‘few thousand’ when I include radiators, pipework, new consumer unit, Open Energy Monitor monitoring equipment.
Yes, my heating is on 24/7 with average room temperature of 19.6.
I don’t have solar or battery, and am on Octopus Agile tariff which has been a bit scary this winter. Cheapest tariff for me at the moment is probably Ovo Heat Pump Plus, but I’m going to hang on with Octopus for the time being. For my usage spread over second half of 2024, Agile was 5% cheaper than Cosy.
I’m hopeful that it won’t be too long before gap between gas and electricity unit price starts to reduce, and then the cost of running an averagely installed heat pump will be comparable to a gas boiler, and for the lucky ones with a better than average install the extra capital cost will be balanced by cheaper running costs.
For now I’m concentrating on trying to get heat losses down through fabric improvements.
Reading this thread again I see that you are on a tariff I’ve never heard of. I am an octopus customer and it doesn’t come up on my list. What price(s) do you pay?
Octopus has a tariff aimed specifically at heat pump owners called “Cosy”, other tariffs are, of course, available. Have you looked into changing your tariff as part of the heat pump process?
Yes Ive put an Octopus day/night tariff into my calculations - think I meant to say Octopus Flux (not Plus.) We’re not on it now, just a standard 12month fix.
Thanks Sian, v useful. That’s quite a good COP you’ve got there! My ‘few thousand’ refers to the ASHP install only; I dont think we’ll need new pipework, and Im working thru upgrading radiators anyway, since that’s better for low-flow-temp heating, whether by gas or HP.
We’ve had our ASHP coming up for 5yrs and we have been averaging a COP of 3.2 over that period. Our previous WB Greenstar boiler was achieving something like 86-88% efficiency. So our ASHP is effectively 3.6-3.7x more efficient than the gas boiler. Despite this we are still saving money.
This because we are making good use of Octoupus electricity tariffs and also we are more comfortable at a pretty much constant ~19.5C daytime and 17.5-18C night-time (set-back) compared to the over-night off period we had with the boiler. With the boiler over-night temps in the house could dip to ~15-16C and then it would have to work quite hard to get back to ~19-20C. The gas boiler (on a simple on-off room stat) would often over-shoot the target temp, so the living room would cycle between ~19-21.5C during the day. Those fluctuations might seen small but during the over-shoot periods the boiler would (unnecessarily or inadvertently) use more gas.
Finally I think in hot water heating the boiler was probably only achieving 75% or less efficiency and we had to wait quite a while for hot water to come our of the taps or shower at a steady temp. Again it would “over-shoot” sending some hot water out after waiting say 30secs then go cold again for perhaps another 30secs and finally settling down to a constant supply of HW after about 1min of waiting. This represented wasted gas.
So despite only having a sCOP of ~3.2 we are saving ~£220-£270 per year and this prior to having a home battery installed. Now with a home battery we are now saving £500-600.
Here is an energy analysis of our heating+HW for the last 7yrs (gas boiler removed Apr-2021)
And here is a tabular version for some of the annual data - note we had gas hob removed Nov-2022 so there was some residual gas use up until then. We then had gas meter removed and stopped paying standing charges for gas.
Thanks Dom, very informative. Re costs, we’re probably not comparing like-with-like here, as your ‘baseline’ was an old-style gas boiler+cylinder set-up, vs ours which is a gas combi-boiler, supposedly running more efficiently… However, despite that!, I note your baseline gas usage used to be 6946kWhr, vs our current usage of a bit under 9,000… Anyway, great to see the details of how a good set-up is performing, thanks.
Our previous gas boiler was a condensing (not old style) combi boiler. I suspect many boiler do not perform as well as advertised or as they might in theoretically ideal circumstances. We had worked hard to get our gas use down over the years and our final years gas use figures reflect that effort.
The logical conclusion of optimising house and system for low gas usage is to realise that you are reaching the limits for gas but interning the zone for heat pump efficiency.
That was the stage I was at when the BUS came along. That and my boiler was getting past its age for reliability. Finding the right heat pump takes a lot more time than finding a boiler that will just do, so I didn’t want to find myself making an emergency purchase if the boiler failed.
I hadn’t used gas for anything else for years, so I had the added bonus of no longer paying the gas standing charge. That buys a lot of electricity at my current rate of 7p/kWh, incl. VAT.
A lot of work went into getting my heat pump settings as they are. When the time comes for a replacement it would be nice if the settings could be downloaded from the old heat pump to the new one. It can’t be done now but hopefully in x years it will be routine.
We do it for phones and other devices. I’m sure it will come.
The other conclusion is that cost savings are slightly complex and there a number of factors at play, not just the sCOP and the standard units rates for gas/elec.
If you have solar PV (as we do) and/or a home battery and you are able to take advantage of favorable electricity tariffs then savings can be had at lower sCOP values than you might also think.
You also have to factor in that no gas boiler is 100% efficient so the sCOP multiplier needs to combined with the additional benefits of not having a less than 100% fossil fuel burner. Then last but not least if you are able to get rid of gas completely then you should ultimately be able to get rid of the standing charge for gas and hence make that saving.
Obviously this complexity is in itself slightly off-putting for the potential customer but it pays to be as well informed as possible. It also pays to keep an eye on how well any heating system is performing once it has been installed and to see if any further optimisation is possible at @Tim_Gilbert suggests.
Indeed. My figures are biased because, as I mentioned above, I pay 7p/kWh for most of my electricity, so my fuel cost is not far off the price of gas, which I think is about 6.5p/kWh plus standing charge. This means I would be better off on even an inefficient heat pump than a super efficient boiler.
For transparency, I have solar PV, battery, EV and of course, a heat pump. I am on Octopus IOG tariff.
While the “Fabric Fifth” group did get a bit of momentum a couple of years ago, I think the divergence of electricity and gas prices has proved why Fabric First is the better fundamental solution to the problem.
Mo’ energy, mo’ problems…
Completely unaffordable to 95% of the population of course, but it’s the purists’ choice.
In principle any home can be heated with a heat pump. It might have to be the size of a small car, but it can be done. I sympathise with the fabric fifth brigade as it is a quick route to decarbonisation if we can generate enough renewable electricity but it puts a huge strain on the grid.
My concern with the method is that you fit a big heat pump to a house, retrofit the house and then your big heat pump is no longer appropriate but a liability.
My four bedroom house is heated by a 6kW heat pump (max 2kW input). 4 years previously I was told I needed a 10-11kW unit. In a few years I hope to need an even smaller one. It is difficult to find the right time to change over.
In my case the deciding factor was that my boiler was becoming unreliable and I didn’t want to have to make an emergency decision to replace it.
Anyhow, I’ve calmed down a bit now, since starting this morosely-headlined thread… Something I didn’t mention is, the current plan is to install a heat battery, instead of a cylinder - this would ‘mobilise’ around 800kWhrs ¶ of PV electricity which is currently wasted (wrt the house) into the grid (for some pittance.) Bit difficult to price that up, but probably worth £150- £200pa.
) Isn’t a good estimate of what we’d need approximately given by